The present invention relates to a method for heat treatment and sintering of ceramic tubes mainly composed of zirconia, having an inside diameter less than a few millimeters, with a wall thickness of a few tenths of a millimeter and length of a few centimeters. It particularly concerns a method making it possible to obtain such tubes which are highly accurate as regards their straightness directly after sintering, without the need for subsequent machining. Such tubes are notably used as sleeves or ferrules for assembling fiber optic connectors.
It is known to produce ceramic tubes having an inside diameter less than a few millimeters, a wall thickness of a few tenths of a millimeter and a length of a few centimeters by a method in which a pre-form consisting of a mixture of ceramic powder and organic binders is shaped by isostatic pressing or injection, followed by heat treatment in order to drive out binders and sinter the ceramic material to densify and consolidate it.
The inside surface of a tube pre-form obtained after this isostatic pressing or injection step is very close to a perfect cylinder as it results from molding the mixture of ceramic powder and binder on a metal core rod which can be readily machined with a high degree of accuracy. Cross-sections of the inside surface of the tube pre-form, perpendicular to its longitudinal axis, are circles. Sections of this inside surface on planes passing through said axis are straight lines which generate the cylinder constituting said inside surface.
During the sintering heat treatment, the tube pre-forms undergo several transformations: the organic binders they contain are volatilized by evaporation or oxidation and the mineral part of the tube becomes densified and consolidated, leading to shrinkage which can be up to 15% to 25% in each direction. Depending on the dimensions and the respective positions of the various parts of refractory material used for positioning the tube pre-forms in the oven and, depending on the heat treatment program, such shrinkage may or may not be accompanied by deformation. In particular, the pre-form, which was rectilinear, bends, thus no longer being straight, while sections of its inside surface, perpendicular to its major length, remain circles of constant diameter from one end to the other. The extent to which the preform is out-of-true, or its straightness defect is measured over its total length by introducing different diameter cylindrical gauges into the pre-form: the straightness defect is defined as being the difference between the diameter of the inside cross-section of the part and the diameter of the largest cylindrical gauge which freely enters the part over all its length. This also corresponds to the fact that if d is the straightness error over a length L, the intersection of the generally cylindrical inside surface of the tube and a plane passing through the axis of this cylinder is a curve which, over a length L, does not deviate by more than d/2 from a straight line, in each direction.
When known sintering methods are used to obtain a tube having the dimensions of an order of magnitude cited above, and of which the exact shape is highly accurate as regards straightness of its inside surface, it is necessary, in view of such deformation, to perform a supplementary machining operation on this surface after sintering, which is difficult in view of the small inside diameter involved, and expensive.